Arrhythmias of the heart, such as fibrillation, are well known to those familiar with the art. Localized or diffuse lesions of the myocardium, which may result from any one of various reasons, often lead to a pronounced dispersion of refractivity. As a result, under certain circumstances the heart does not experience a total, simultaneous depolarization, but rather there results an unequal repolarization which has a dispersed vulnerable phase. A depolarization excitation interspersed during this vulnerable phase finally leads to electrical fragmentation, and a consequential inception of ventricular fibrillation.
It is known that the proper application of an electrical shock to the heart can change a fibrillating heart back to synchronous action of all myocardial fibers, that is, the heart can be defibrillated. Defibrillation by electrical impulses to the heart is due to a regular development of propagation of electrical excitation by means of simultaneous switching of all myocardial fibers that have gone out of step to cause the arrhythmia. Many defibrillation devices are known in the prior art for providing a defibrillation pulse after the arrhythmia has commenced.
However, it has become apparent that electrical defibrillation is not an ideal means of therapy for arrhythmia problems. First of all, it is not immediately available in most cases, and even where implantable defibrillation devices are used, they provide stimulation signals only after the dangerous condition of arrhythmia already exists. Further, though implantable defibrillators were developed to eliminate existing ventricular fibrillation as rapidly as possible, they can do so only after detection of the actual state of fibrillation; and because of the high power requirements of the electrical shocks required to defibrillate, the operating time of such implantable defibrillators is highly limited. Further, even after detecting the advent of fibrillation, such prior art defibrillators require a discreet period of charge time before providing a defibrillation shock.
To overcome the problems discussed above, the apparatus of this invention was designed to prevent defibrillation by sensing heart depolarizations and providing cardiac stimulation level pulses to the heart to overcome electrical fragmentation before it causes defibrillation or other arrhythmias.